Prior drop-on-demand ink jet print heads typically eject ink drops of a single volume that produce on a print medium dots of ink sized to provide printing at a given resolution, such as 12 dots per millimeter (300 dots per inch (dpi)). Single dot size printing is acceptable for most text and graphics printing applications that do not require high image quality. Higher image quality, such as "photographic" image quality, normally requires higher resolution, which slows the print speed. Image quality may also be improved by adding ink color densities, which undesirably requires an increase in the number of jets in the print head.
Another technique for improving image quality is to modulate the reflectance, or gray scale, of the dots forming the image. In single dot size printing, the average reflectance of an image portion is typically modulated by a process referred to as "dithering." In a dithering process the perceived intensity of an array of dots is modulated by selectively printing the array at a predetermined dot density. For example, if a 50 percent local average reflectance is desired, half of the dots in the array are printed. A "checker-board" pattern provides the most uniform appearing 50 percent local average reflectance. Multiple dither pattern dot densities are possible to provide a wide range of reflectance levels.
However, dithering necessitates a trade off between the number of possible reflectance levels and the dot array area required to achieve those levels. Eight-by-eight dot array dithering in a printer having 12 dot per millimeter resolution results in an effective gray scale resolution as low as 3 dots per millimeter (75 dots per inch). Gray scale images printed with such dither array patterns often appear grainy and suffer from poor image quality, especially in areas having a low optical density.
One approach to improving the quality of gray scale images printed with dithering is ink dot size modulation, also referred to as drop volume and drop mass modulation. Ink drop volume modulation entails controlling the volume of each drop of ink ejected by the ink jet print head. Drop volume modulation advantageously provides greater effective printing resolution without sacrificing print speed. For example, an image printed with two dot sizes at 12 dots per millimeter (300 dots per inch) resolution may have a better appearance than the same image printed with one dot size at 24 dots per millimeter (600 dots per inch) resolution. This increase in effective resolution is possible because using two or more dot sizes in low optical density areas increases the dot density (dots/area), which in turn decreases graininess.
There are previously known apparatus and methods for modulating the volume of ink drops ejected from an ink jet print head. U.S. Pat. No. 3,946,398 for a METHOD AND APPARATUS FOR RECORDING WITH WRITING FLUIDS AND DROP PROJECTION MEANS THEREFORE describes a variable drop volume drop-on-demand ink jet head that ejects ink drops in response to pressure pulses developed in an ink pressure chamber by a piezoelectric transducer (hereafter referred to as a "PZT"). Drop volume modulation entails varying an amount of electrical waveform energy applied to the PZT for the generation of each pressure pulse. However, it is noted that varying the drop volume may also vary the drop ejection velocity and result in drop landing position errors. Constant drop volume, therefore, is taught as a way of maintaining image quality. The drop ejection rate is also limited to about 3000 drops per second (3 kHz), a rate that is slow compared to typical printing speed requirements.
U.S. Pat. No. 5,124,716 for a METHOD AND APPARATUS FOR PRINTING WITH INK DROPS OF VARYING SIZES USING A DROP-ON-DEMAND INK JET PRINT HEAD, assigned to the assignee of the present invention, and U.S. Pat. No. 4,639,735 for APPARATUS FOR DRIVING LIQUID JET HEAD describe circuits and PZT drive waveforms suitable for ejecting ink drops smaller than an ink jet orifice diameter. However, a separate drive waveform must be generated and applied to the PZT for each different drop size. The waveform generating componentry required to produce the multiple waveforms is undesirably complex and adds additional cost to the printer.
Another approach to modulating drop volume is disclosed in U.S. Pat. No. 4,746,935 for a MULTITONE INK JET PRINTER AND METHOD OF OPERATION. This describes an ink jet print head having multiple orifice sizes, each optimized to eject a particular drop volume. Of course, such a print head is significantly more complex than a single size orifice print head and still requires a very small orifice to produce the smallest drop volume.
U.S. Patent No. 5,689,291 for a METHOD AND APPARATUS FOR PRODUCING DOT SIZE MODULATED INK JET PRINTING, assigned to the assignee of the present application, provides multiple PZT drive waveforms for producing various ink drop volumes. The various ejected ink drop volumes have substantially the same ejection velocity over a range of drop ejection repetition rates. As with other previous systems, a different drive waveform must be generated and applied to the PZT for each drop volume desired.
What is needed, therefore, is a simple and inexpensive ink jet print head system that provides high-resolution drop volume modulation without requiring multiple drive waveforms and the associated generation and control componentry, and without sacrificing print speed. This need is met by the apparatus and method of the present invention.